U.S. Pat. No. 8,331,869 describes systems and methods for over-the-air performance testing of wireless devices with multiple antennas. This class of system, referred to as a boundary array system, reproduces a radiated near-field environment that appears to the device in the test volume as though it originated in the far field and had the multipath characteristics of a chosen emulated environment.
FIG. 1 is a typical multiple input multiple output (MIMO) boundary array configuration 10 for a test of a device under test (DUT) 22, showing boundary array antennas 12 in an anechoic chamber 14 with a wireless communication tester 16 connected through a spatial channel emulator 18 and amplifiers 20. Splitters 28 may be interposed between the wireless communication tester 16 and the spatial channel emulator 18. Some configurations require multiple individual channel emulators synchronized together to produce sufficient output channels to drive all of the antenna elements in the chamber. The test configuration of FIG. 1 is typically used to evaluate the receiver performance of DUT 22. When the DUT 22 is a cellular phone, for example, the test configuration of FIG. 1 would evaluate the downlink signal from the base station to the mobile phone. When the DUT 22 is a base station, for example, the test configuration of FIG. 1 is would evaluate the uplink signal from the mobile phone to the base station. For simplicity, this document will refer to the DUT receiver test configuration as the downlink, and the DUT transmit test configuration as the uplink. The configuration of FIG. 1 is uni-directional for simplicity. Bi-directional systems are also employed.
The device under test (DUT) 22 is positioned on a positioner, such as a turntable, within a test volume of the anechoic chamber 14 that is isolated from the environment exterior to the anechoic chamber 14 by RF absorber lined walls, floor and ceiling. The array of antennas 12 radiate electromagnetic energy (radio waves) toward the DUT in a variety of directions. The radiated signals from each of the antennas 12 have various impairments (delay spread, Doppler, interference, etc.) applied through spatial channel emulator 18 to simulate multipath fading in a real world environment.
The various impairments are introduced into signals received from the wireless communication tester 16 by one or more spatial channel emulators 18 that digitize the received signals. The digitized received signals are delayed and weighted in amplitude by the spatial channel emulator 18. More particularly, the spatial channel emulator 18 may add multipath delay, delay spread, fading, interference, and other impairments common in typical radiated communication paths, and then converts the result to analog signals and up-converts the result to a radio frequency, RF. Thus, each output of the spatial channel emulators 18 may be the sum of multiple replicas of the input signal delayed and weighted according to a channel model definition, and will vary in time based on a motion definition that models relative motion of the DUT 22 or an intervening reflector. Doppler frequency shift may also be introduced arising from the relative motion. Interference may also be introduced by adding additive white Gaussian noise (AWGN) or other noise as well as by injecting specific interfering signals. The full panoply of channel effects emulated by the channel emulator are referred to herein collectively as impairments.
In a typical configuration, the number of inputs to the spatial channel emulator 18 may be different from the number of outputs of the spatial channel emulator 18. Splitters 28 may be interposed between the wireless communication tester 16 and the spatial channel emulator 18. Each output of the spatial channel emulator 18 is amplified by a power amplifier 20 and directed on a path, typically provided by cables, to an antenna 12. The spatial channel emulator emulates a plurality of channels, each channel being associated with a different one of the antennas 12.
Amplification is required between the spatial channel emulator 18 and the antennas 12 in order to produce sufficient radiated power to be received by the DUT on the downlink and to amplify the weak signals received from the DUT to be well above the receiver sensitivity of the channel emulator on the uplink. The wireless communication tester 16 emulates an end of a radio link opposite the DUT. The uplink is the path of signal propagation from the DUT 22 to the wireless communication tester 16 (these paths not being shown in FIG. 1).
The wireless communication tester generates signals according to a communication protocol of the DUT. For example, the wireless communication tester 16 may generate transmit signals that are formatted for long term evolution (LTE) signaling, and may receive signals from the DUT that are also formatted for LTE signaling. Other communication protocols, such as Wi-Fi, may be employed by the wireless communication tester 16. Also shown is a communication antenna 24 coupled to a low noise amplifier (LNA) which is connected to the wireless communication tester 16. The purpose of the communication antenna 24 is to provide an alternate, un-faded and potentially low loss communication path between the DUT and the communication tester for signals that are unrelated to the metric being tested on the DUT (e.g. closed loop feedback of a digital error rate during a receiver sensitivity test) in order to maintain the full communication link.
FIG. 2 is an implementation of one example of an RF channel emulator 18 that includes emulator receivers (vector signal analyzers) 30 and emulator transmitters (vector signal generators) 32 around a digital signal processing channel emulator core 34. In the channel emulator core, each signal may be impaired and added to other signals to produce impaired signals in order to simulate the effects of one or more signals propagating over the air, being reflected off of obstacles such as buildings, and arriving at the DUT with different amplitudes and phases. Doppler shift may also be introduced by the channel emulator core 34.
Common components of the emulator receivers 30 are shown in FIG. 3. A low noise amplifier 36 receives an RF signal, possibly having a low SNR, and amplifies the RF signal and optionally passes the amplified RF signal to a further amplification stage that includes a variable gain amplifier 38. The amplified RF signal is down-converted in a mixer 40 with a local oscillator (LO) signal from an LO 42 to produce an intermediate frequency (IF) or baseband signal that is filtered by a filter 44, and possibly further amplified by an amplifier 46. The signal output of the amplifier 46 is an analog signal which may be converted to a digital signal by an analog to digital converter (ADC) 48.
Common components of the emulator transmitters 32 are shown in FIG. 4 that include a digital to analog converter (DAC) 50, a filter 52, and an amplifier 54. The signal that passes through these components may be at baseband or at an intermediate frequency. The signal is then mixed in a mixer 56 with a local oscillator signal from an LO 58. The output of the mixer is an RF signal that may be further amplified by a variable gain amplifier (VGA) 60.
FIG. 5 is a single emulated downlink channel between a wireless communication tester 16 and a DUT 22. (For simplicity, we shall call the direction of propagation of FIG. 5 the downlink direction). In the downlink, the wireless communication tester 16 generates transmit signals to be received over the air by the wireless device, DUT 22. The transmit signals from the wireless communication tester are routed to the spatial channel emulator 18a by signal routing 62a, which may be, for example, coaxial cables, and switches that would enable switching to calibration paths or other test paths (not shown) in order to provide flexibility to alter the configuration as desired. Note that there may be any number of RF paths between the wireless communication tester 16 and the spatial channel emulator 18 for multiple input multiple output (MIMO) or diversity testing, all of which are combined into a single RF path for each antenna element at the output of the spatial channel emulator 18.
The spatial channel emulator 18a replicates each signal received from the communication tester 16, impairs each replica in a different way, and combines the impaired replicas to produce an impaired signal of the channel. Note that the applied impairments may simulate multipath effects as well as Doppler shift and other time and frequency dependent effects. The impaired signal output by the spatial channel emulator 18a is an RF signal that is coupled by signal routing 62b to an amplifier 64 to be amplified. The output of the amplifier 64 is routed to the anechoic chamber 14 to the antenna 12 by signal routing 62c. The antenna 12 radiates the impaired signal to the device under test 22. Note that the signals carried by the signal routing, herein referred to collectively as signal routing 62, are RF signals, and thus, may exhibit significant losses.
FIG. 6 is a single emulated uplink channel between a DUT 22 and a wireless communication tester 16. (For simplicity, we shall call the direction of propagation of FIG. 6 the uplink direction). Signals radiated by the DUT 22 are received by the antenna 12 which converts the electromagnetic radiation (radio waves) to an RF signal which is amplified by a low noise amplifier (LNA) 36. The RF output of the LNA 36 is coupled out of the anechoic chamber 14 to the spatial channel emulator 18b by signal routing 62d. The spatial channel emulator 18b may apply different impairments to replicas of the received RF signal to form impaired signals to simulate a multipath, Doppler-shifted environment. The output of the spatial channel emulator 18b is at least one output signal that is coupled by signal routing 62e to the wireless communication tester 16.
Note once again that the signal routing 62 carry RF signals, and thus, the signal routing 62 introduce significant losses. As with the downlink chain of FIG. 5, in the uplink configuration of FIG. 6, there may be any number of RF paths between the spatial channel emulator 18 and the wireless communication tester 16 for MIMO or diversity testing. The RF paths may be derived from a single RF signal at the input of the spatial channel emulator 18.
Note that bidirectional channel emulation can be performed with two separate, synchronized channel emulators 18a and 18b or as a single bi-directionally configured unit. Each channel emulator block may also be realized by a plurality of channel emulators, referred to herein collectively as spatial channel emulators 18.
While the boundary array technique is a powerful mechanism that can theoretically produce any desired RF environment, the capabilities of currently available RF test equipment provide physical, practical, and financial limits to what can be achieved with the system.
The ability to produce a uniquely correlated spatial distribution within the test volume is governed by the same Nyquist theorem limitations of near-to-far-field conversion, whereby a spherical surface surrounding the DUT should have at least two sampling points (antenna directions) per wavelength along the surface of the sphere. The larger the antenna separation or general RF interactive region on the DUT, the more active antennas are needed in the boundary array in order to produce the proper RF environmental conditions.
In addition to the physical constraints of the antenna size around the perimeter of the test volume, which forces a larger array diameter as the number of antennas 12 increases, the number of amplifier and channel emulator resources required increases by as much as four times the number of antenna locations. Since each antenna location may be called upon to support two antenna elements in orthogonal polarizations (i.e. a dual polarized antenna), and assuming bi-directional communication, each antenna location requires four amplifiers connected to two channel emulator transmitters and two channel emulator receivers remote from the antenna locations. In addition, for full spherical coverage, the number of required antennas increases as the square of the frequency to be tested multiplied by the maximal radial extent (MRE) dimension of the DUT, i.e. N∝(f r)2 or N∝(r/λ)2, where r is the radial dimension, N is the number of antennas, and A is the wavelength. Stated simply, as the test frequency and/or DUT size increases, the number of required antennas increases.
Since RF channel emulators 18 were originally designed for conducted testing of radio transmitters and receivers, adapting them for use in over-the-air testing conditions requires the addition of amplification to overcome the losses associated with RF cables, antenna efficiencies of both the boundary array antennas 12 and antennas of the DUT 22, and free-space path losses due to the range length. Since the power amplifiers 20 are independent of the power control of the spatial channel emulator 18, they must provide highly linear performance in order to generate the expected power levels within the test volume.
Likewise, since power control occurs before amplification on the downlink, the desired signal level moves closer to the instrumentation noise floor and then both signal and noise are amplified and injected into the chamber, where the instrumentation noise may become a significant portion of the signal-to-noise ratio (SNR) seen at the DUT receiver. The noise figure of the power amplifier is also added to the noise of the channel emulator and other instrumentation, thereby decreasing the SNR.
Similarly, on the uplink, the signal received at the boundary array antenna 12 from the DUT 22 is well below the signal level expected at the input to the spatial channel emulator 18, so low noise amplification is required to boost it above the receiver sensitivity of the spatial channel emulator 18. Since cable losses associated with bringing the signal out of the anechoic chamber 14 to the spatial channel emulator 18 input add to the loss, the resulting negative impact on signal to noise ratio is increased.
Also, bi-directional communication where both downlink and uplink signals are present simultaneously requires the introduction of some form of isolation to ensure that the high power output of the downlink amplifier is not coupled into the highly sensitive input of the lower noise amplifier. Any cross coupling between the two amplifiers can severely degrade system performance and is highly likely to cause damage on the uplink side, either at the amplifier and/or the input to the channel emulator 18b. 
Since conventional spatial channel emulators 18 are large rack mount pieces of test equipment that reside outside the shielded anechoic chamber 14, as the number of antenna locations increases, not only does the range length increase, but the required length of all cables between the channel emulators 18 and amplifiers 20 and the boundary array antennas 12 generally increases by at least π times the increase in radius. While the free-space path loss increases logarithmically with the increase in radius, the loss of an RF cable is a linear function of the cable length. Thus, eventually the cable losses can dominate the losses of the system as the system is scaled up to include more channels.
Conversely, in suitable instrumentation amplification, there is an upper limit to the output power of a single power transistor, so that increasing the amplification to overcome additional path loss becomes a problem of parallel amplification rather than series amplification, with the associated complexities of combining the power at the output. The result is that the associated size, cost, heat generation, etc. for the larger amplifiers grows exponentially as the linear output power increases. Finally, the number of required RF cables also increases by the same four times the number of probe positions that the amplifiers and channel emulation must increase.
As to the wireless communication tester 16, the process of generating an RF signal and then tuning and digitizing it in order to perform the channel emulation via the spatial channel emulator 18 introduces additional error and uncertainty into the signals for both uplink and downlink.
Thus, one problem with existing systems is the RF path loss associated with the distances involved and the amplification required to overcome these losses. The use of existing centralized RF channel emulators designed for conducted testing coupled with the expensive high power amplifiers needed to overcome this path loss results in most of the expense of the amplification being spent to heat up the RF cables due to internal losses.